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3 - Installation
32
Minimizing the load lead resistance reduces voltage drops V
LD+
and V
LD-
. ∆V can be further minimized
by decreasing the resistance of the sense leads (R
S+
and R
S-
) as much as possible. In situation where ∆V
cannot be minimized any further, it may be compensated by programming a negative output resistance as
previously discussed.
Maintaining Stability while Remote Sensing
The remote sense bandwidth and slew rate of standard dc power sources are adequate for compensating
for load lead voltage drop for slow to moderate rates of load changes. However, the high pulsed current
draw of digital cellular phones presents a challenge to standard dc power sources operating in remote
sense mode. Their bandwidth and slew rate are not adequate for dealing with the 0.05 to 0.2 amp/µs slew
rates imposed by these devices. A large voltage transient occurs at the load, due to the inability of the dc
source to keep up with the rate of load change.
The dc source effectively compensates for load lead voltage drops resulting from very high slew rate load
current transitions. This keeps the remotely sensed output voltage at a relatively constant level. For 0.05
amp/µs to 0.2 amp/µs slew rate loading in typical test applications, the transient voltage is reduced more
than an order of magnitude over that of other standard dc sources.
Open Sense Lead Protection
The main output (output 1) of the dc source has built-in open sense protection circuitry that detects if
there is an open in either the positive or the negative remote sense lead or load lead path. For battery
powered devices, undetected open sense connections can cause incorrect battery charger calibration,
incorrect test results due to erroneous voltage settings, and low voltage phone shutdown due to a large
transient voltage drop.
To enable open sense lead detection from the front panel, press the
Output key, use ' to scroll to
SENS:PROT, press
&
&&
& to select ON, then press Enter. To have the unit turn on with open sense detection
enabled, save this state in location 0 and set the power-on state to RCL 0.
When this circuit is enabled, the sense and load leads are checked every time the output transitions
from disabled to enabled (off to on). If a lead opens while the output is enabled, this will not be
detected immediately by the open sense circuit. However, the output voltage will increase or decrease,
depending on which one of the leads is open. Turning the output off, then on again, will cause the unit to
check the output sense and load leads and determine if a sense lead is open.
If the open sense lead protection circuit detects an open sense lead, the Prot annunciator on the front
panel turns on and the output turns off. Bit 5 in the Questionable Status Registers is also set (see chapter
7 under "Programming the Status Registers"). On the front panel, press the Prot key, and one of the
following error messages will be reported on the front panel:
Message
Description
+ sense open
Positive sense or load lead is open
- sense open
Negative sense or load lead is open
+/- sense open
Both positive and negative sense or load leads are open
sense open
Incorrect resistance reading on the sense or load leads. This may be caused by an
external power source paralleled with the output, or in rare instances, by the voltage
being out of calibration.